Non-reflective internal lossy terminations for slow wave circuits and tubes using same



Aprll 8', 1969 J E, N HEL 3,437,866

NON-REFLECTIVE INTERNAL LQSSY TERMINATIONS FOR SLOW WAVE CIRCUITS AND TUBES USING SAME Filed June 14, 1966 1 sheet. of 2 PIC-3.3

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p 8, 1969 J. E. HENTSCHEL. 6

NON-REFLECTIVE INTERNAL LOSSY' TERMINATIONS FOR SLOW WAVE CIRCUITS AND TUBES USING SAME Filed June 14, 1966 Sheet 2 of 2 FIG.-|4 1 u FIG.|2|6 n -+|3 52 v 35 FIG.I5

as r 42 5| as 5 INVENTOR. g JOHN TSCHEL BY Q RNEY United States Patent NON-REFLECTIVE INTEEiNAL LOSSY TERMINA- TIONS FOR SLUW WAVE CIRCUITS AND TUBES USING SAME John E. Hentschel, East Brunswick, N.J., assignor to S-F-D Laboratories, Inc, Union, N..I., a corporation of New Jersey Filed June 14, 1966, Ser. No. 557,398 Int. Ci. Hillj 25/34, 1/38, 19/06 U.S. Cl. 315-3.5 8 Claims The present invention relates in general to lossy terminations for slow wave circuits and, more particularly, to an improved non-reflective wave termination for slow Wave circuits which is especially useful for dissipating moderately high average power internally of microwave tubes using same. Such improved terminations are especially useful in microwave amplifier tubes used as noise generators wherein the low power end of the circuit is terminated in a resistive load internally of the tube.

Heretofore, microwave noise tubes have been built wherein the low power end of a slow wave amplifier circuit was terminated internally of the tube in a resistive circuit portion. Typically in such applications the resistive circuit termination was formed by coating the periodic elements of the slow wave circuit with a lossy material such as Kanthol or Aquadag. In other prior art tubes the lossy termination was provided by placing a loss-y member such as carbon impregnated alumina ceramic adjacent the slOW wave circuit such that fields of the circuit were heavily coupled into the lossy member. The problem with these prior art lossy terminations is that they provide insuiiicient loss to the wave energy on the circuit either by being insufiiciently thick such that the Wave energy could travel along the conductive elements beneath the lossy coating or by providing insuflicient coupling to the fields of the slow wave circuit. As a result an inordinate length of slow wave circuit had to be devoted to the lossy termination or insuflicient loss pro vided. Sutficient loss could always be provided by coupling the circuit to an external resistive load, but this expedient was complicated by having to provide impedance matches to the slow wave circuit and by having to provide vacuum tight microwave windows to take the microwave power through the vacuum envelope of the tube to the external load.

In the present invention, the slow wave circuit is terminated internally of the tube by making a few of the terminating elements of the slow wave circuit of a very lossy material having a resistivity greater than 1000' 1() 9cm. Such a material is carbon or carbon impregnated alumina. When the terminating periodic elements of the circuit are made of such high loss material the field configuration for the slow wave circuit are not abruptly changed and therefore the wave energy propagates into the lossy material in a non-reflective manner. Due to the high loss of the material only a few lossy periodic elements are required to give the required loss and therefore the termination is short.

The principal object of the present invention is the provision of improved internally terminated slow wave circuits and tubes using same.

One feature of the present invention is the provision of a lossy termination for a periodic slow wave circuit wherein the terminating periodic elements of the circuit are made of a lossy material having a resistivity greater than 1(l00 10- SZcm., whereby the termination is made non-reflective and sufiiciently lossy such that only a few of such lossy elements are required.

Another feature of the present invention is the same as the preceding feature wherein the slow wave circuit is 3,437,866 Patented Apr. 8, 1969 selected from the class consisting of, helices and topological equivalents, interdigital lines, reactively loaded interdigital lines, helix coupled vane circuits, and strapped bar circuits.

Another feature of the present invention is the same as any one or more of the preceding features wherein the slow wave circuit is employed in a crossed field noise generator tube and the termination is disposed internally of the vacuum envelope of the tube.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a transverse schematic line diagram of a noise tube employing features of the present invention,

FIG. 2 is a sectional view of the structure of FIG. 1' taken along line 22 in the direction of the arrows,

FIG. 3 is a sectional view of the structure of FIG. 2 taken along line 3-3- in the direction of the arrows,

FIG. 4 is a plot of frequency w versus phase shift per section {3 for the slow wave circuit of FIGS. 2 and 3,

FIG. 5 is a sectional view of an alternative slow wave circuit for the tube of FIG. 1 taken along line 55 in the direction of the arrows,

FIG. 6 is a linearized sectional view of the structure of FIG. 5 taken along line 6-6 in the direction of the arrows,

FIG. 7 is a sectional view of the structure of FIG. 5 taken along line 77 in the direction of the arrows,

FIG. 8 is a sectional view of an alternative slow wave circuit for the tube of FIG. 1 taken along line 88 in the direction of the arrows,

FIG. 9 is a sectional view of the structure of FIG. 8 taken along line 99 in the direction of the arrows,

FIG. 10 is a linearized view of the structure of FIG. 8 taken along line 1010 in the direction of the arrows,

FIG. 11 is a transverse schematic diagram of an alternative backward wave noise tube incorporating features of the present invention,

FIG. 12 is a sectional view of the structure of FIG. 11 taken along line 1212 in the direction of the arrows,

FIG. 13 is a sectional view of the structure of FIG. 12 taken along line 13-13 in the direction of the arrows,

FIG. 14 is an to versus 8 diagram for the backward wave circuits of FIGS. 11-13 and 15-16,

FIG. 15 is a sectional view of an alternative slow wave circuit for the tube of FIG. 11 taken along line -1515 in the direction of the arrows,

FIG. 16 is a sectional view of the structure of FIG. 15 taken along line 16-16 in the direction of the arrows, and

FIG. 17 is a perspective view of an alternative reactively loaded interdigital line slow wave circuit of the type depicted in FIGS. 2 and 3 as employed in linear type 0 beam tubes.

Referring now to FIG. 1 there is shown a conventional forward wave crossed field noise tube 1. The tube 1 includes a cylindrical thermionic cathode emitter electrode 2 surrounded by a fundamental forward wave slow wave circuit 3 which is operated at anode DC. potential. The annular space 4 between the slow wave circuit 3 and the cathode 2 defines an electronic interaction region wherein the microwave radio frequency R.F. fields of the slow wave circuit cumulatively interact with the electrons of the stream to produce amplification of microwave energy traveling on the slow wave circuit 3. An axially directed magnetic field B passes through the interaction region 4 in a direction transverse to the electric field produced between the cathode and anode to cause the electrons to circulate around the cathode in a reentrant stream manner as shown by the arrows marked with an e. An evacuated vacuum envelope 5 encloses the anode and cathode.

The anode circuit 3 is severed at 6 to define an input or low power end of the circuit at 7 and a high power end or output terminal at 8. The input terminal end 6 of the circuit 3 is terminated internally of the tube 1 in a non-reflective lossy termination 10.

In operation, with operating voltage applied between anode and cathode, noise power in the electron stream is coupled onto the slow Wave circuit 3 over the pass band of the circuit 3. This noise power is amplified by electronic interaction with the circulating electrons to produce an output noise signal which is extracted from the tube 1 at the output terminal 8 and fed to a suitable utilization device or load, not shown.

The slow wave circuit 3 may take various different forms. A particularly suitable forward wave circuit is the reactively loaded interdigital line as shown in greater detail in FIGS. 2 and 3 and which has a dispersion characteristic as shown in FIG. 4. The reactively loaded interdigital line slow wave circuit is described and claimed in copending US. application 350,504, filed Mar. 9, 1964, abandoned in favor of continuation application Ser. No. 637,007, now issued as US. Patent 3,358,179 and assigned to the same assignee as the present invention. Briefly, the reactively loaded interdigital line circuit 3 includes a pair of elongated conductors 9 and 11 each having a plurality of periodic conductive elements or fingers 12 and 13 projecting transversely therefrom. The fingers 12 and 13 are interdigitated to form a meandering wave path along the circuit 3. In addition, the fingers 12 and 13 are bifurcated with slots 14 to form periodic reactive loading elements 14 in series with each of the conductors 9 and 11. The electron stream e sequentially interacts with the series and shunt voltages developed across slots 14 and between adjacent interdigitated fingers 12 and 13, respectively. The circuit 3 has a pass band from 00 to 01 where m is the high frequency cut off and corresponds to a length along the circuit, as indicated in FIG. 2, which corresponds to 180 of electrical phase shift of the Wave traveling on the circuit. This distance is substantially less than one half a free space wavelength due to the reactive loading produced by the loading slots 14. The low frequency cut off (0 is determined by the circuit support structure. If the circuit 3 is supported on insulators this frequency m is essentially zero frequency. If the circuit 3 is crown supported as shown in FIG. 3 the low frequency cut off w corresponds to a frequency wherein the distance Z is approximately one half a wavelength.

The slow wave circuit 3 of FIGS. 1-3 is terminated in a lossy non-reflective termination by forming one or more of the terminal periodic elements 13' or 12 of the slow wave circuit of a lossy material such as carbon or carbon impregnated ceramic, for example, alumina or I beryllia ceramic. The lossy material has a resistivity greater than 10O0xl0- Qcentimeters. Carbon has a resistivity of 4600x10 Qcm.

In the case of the reactively loaded inter-digital line circuit 3, the terminating periodic element 13 may or may not be bifurcated as desired. If this element 13 is bifurcated a better impedance match to the remainder of the slow wave circuit is obtained but an adequate match is obtained for many purposes if the finger 13' is not bifurcated. More than one finger 12 or 13 of the circuit may be made of the lossy material to form the termination 10. In addition, the effectiveness of a short termination 10 may be enhanced by providing a short circuit reflection a quarter wavelength along the circuit 3 from the start of the termination 10. Since the guide wavelength is increased by the attenuation of the wave energy a shortest possible termination would have the loss concentrated near the start of the termination 10, with low loss material inbetween the concentrated loss and the reflector to descrease the guide wavelength. Such a reflective element is easily provided by the circuit sever 6. Also the extensions 9 and 11' of the two elongated conductor portions 9 and 11 may be made of the lossy material to add further loss to the termination 9 if desired. In addition, the back wall structure 5 may be made of lossy material to further enhance the loss of the termination 10 since it forms a part of the circuit 3. If the terminating elements 13, 9 and 11 are made of carbon impregnated ceramic the ceramic may be afiixed to the remainder of the circuit by conventional metallizing and brazing techniques and then carbon impregnated using conventional techniques. The brazed carbon impregnated ceramic termination 10 will provide good thermal conduction to the body 5 of the tube 1 for cooling by the conventional cooling techniques such as fluid coolant channels used for cooling elements of the tube 1. If the terminating element is made of carbon it may be aflixed to the remainder of the slow wave circuit by a suitable cement or mechanically held in position by metal retaining tabs or a jacket crimped over the base portion of the lossy element 13.

Referring now to FIGS. 5-7 there is shown an alternative forward wave slow wave circuit 3 including the lossy termination 10 of the present invention. More particularly, circuit 3' com-prises a helix formed by slotting a tubular conductor of reactangular cross section. For circular tube geometries the tubular conductor is formed into a toroid before slotting. Three sides of the tube are slotted with an array of transverse slots 15 at right angles to the axis of the tubular conductor and the remaining side of the tube, remote from the electronic interaction side of the circuit 3, is slotted with an array of slanted slots 16 intersecting adjacent transverse slots 15 and forming interconnecting conductors 17 advancing the helix from turn to turn. Each turn of the helix circuit 3' defines a periodic element of the helix circuit 3. The helix circuit 3' is held in position by a pair of dielectric slabs 18, as of alumina ceramic, overlaying the top and bottom surfaces of the helix circuit 3'. The slabs 18 are held between a pair of conductor portions forming a part of the surrounding vacuum envelope 5 and support structure. The circuit sever 6 is formed by an unslotted region of the tubular conductor.

The lossy termination 10 for the helix circuit 3, or topological equivalents thereof, is provided by making one or more terminal turns 19 of the helix of the lossy material as aforedescribed. Also as aforedescribed an effective short termination may be formed by making the circuit length along the helix circuit one-quarter wavelength from the start of the lossy material of the termination to the reflective short circuit termination or sever 6 formed by the unslotted portion of the tubular conductor. In another embodiment of the present invention, which would not be as non-reflective, the circuit sever 6 could be formed of the lossy material with the helix circuit being made of conductive material up to the junction between the circuit 3 and the circuit sever 6. The helix circuit 3 and tubes using same are described and claimed in copending US. application 406,305, filed Oct. 26, 1964, now issued as U.S. Patent 3,376,463, and assigned to the same assignee as the present invention.

Referring now to FIGS. 810 there is shown another forward wave slow wave circuit employing circuit termination features of the present invention. This circuit 3" may be considered as a helix coupled vane circuit or as a stub supported helix circuit. The helix portion 22 of the circuit 3" is formed by a slotted tube in the same manner as the helix circuit 3, previously described with regard to FIGS. 5-7. Each turn of the helix 22 is supported from and connected to the conductive support wall structure 5 via the intermediary of a conductive stub member 23. The conductive stubs 23 form an array of periodic elements for developing radio frequency voltages therebetween for interaction with the electron stream e. The stubs 23 preferably have width dimensions to, near their points of connection to the helix 22, which are approximately equal to the similar width dimensions w of the conductor forming the helix 22. The stubs 23 preferably have a length l near a quarter wavelength at the center of the pass band of the slow wave circuit formed by the helix coupled stubs or vanes 23.

The lossy termination for the slow wave circuit of FIGS. 8-10 is formed by making the terminal stub 23' and helix turn elements 19 out of the lossy material as aforedescribed. Also the sever 6 may be made conductive and provided one-quarter of a wavelength along the circuit from the junction of the conductive portion of the circuit with the lossy portion of the circuit as aforedescribed. Also the circuit sever portion may be made of lossy material with the conductive circuit extending up to the lossy sever member 6, but this latter alternative will be more wave reflective than the other embodiments. The helix coupled vane circuit is described and claimed in copending US. application 454,140, filed May 7, 1965, now issued as US. Patent 3,387,170, and assigned to the same assignee as the present invention. The termination described with regard to the helix coupled vane circuit of FIGS. 8-10 is equally applicable to other helix coupled circuits such as, for example, dual helix coupled vane circuits, helix coupled bar circuits and dual helix coupled bar circuits described and claimed in the aforementioned application 454,140.

Referring now to FIGS. 1113 there is shown a backward wave noise generator tube 25 incorporating features of the present invention. In this case the anode circuit 3 is a backward wave circuit having a dispersion characteristic as shown in FIG. 14. The lossy circuit termination 10 is provided at the downstream end of the severed circuit 3. Noise on the spokes of space charge in the interaction region 4 is amplified by the backward wave interaction process to produce noise power output which is extracted from the circuit 3 at the output terminal 26 disposed at the upstream end of the circuit 3.

One backward wave type of slow wave circuit 3 is the interdigital line circuit 27 of FIGS. 12 and 13. The circuit 27 comprises a pair of elongated conductors 9 and 11, as previously described with regard to FIGS. 2 and 3, having interdigitated conductive fingers 12 and 13 except that fingers are not bifurcated. The circuit 27 is provided with a lossy termination 10 in the same manner as described with regard to FIGS. 2 and 3. The circuit 27 may be crown supported from the envelope structure 5 as previously described or, in addition, may also be stub supported as shown in FIG. 13.

In a stub supported version, the stubs 28 are conductive over the conductive portion of the circuit and are made of lossy material over the lossy termination portion of the circuit 27. The stubs 28 interconnect the interdigitated fingers 12 and 13 with the support envelope structure 5 to provide the circuit with additional thermal capacity and mechanical strength. The stubs 28 preferably have a width w, which is less than the width of the fingers o and preferably have a length which is as long or longer than the finger length I The stubs 28 tend to lower the interaction impedance of the circuit. In the termination section 10 the stubs 28' are preferably made of the lossy material. The stub supported interdigital line 27 forms the subject matter of and is claimed in US. application 350,516, filed Mar. 9, 1964, now issued as US. Patent 3,361,926, and assigned to the same assignee as the present invention.

Referring now to FIGS. 15 and 16 there is shown an alternative strapped bar type of backward wave slow wave circuit 3. In this embodiment the bar circuit 29 comprises an array of parallel conductive bars 31 which are approximately a half an electrical wavelength long at the upper cut off frequency of the circuit 29. The bars 31 are shorted together at their ends by a pair of elongated conductor portions 32 and 33 to define an array of half wave slot resonators 34 in the spacer between adjacent bars 31. A pair of conductive straps 35 and 36 extend along the array of bars 31 with each strap being connected, as indicated by the Xs, to alternate ones of the bars 31 such that adjacent bars 31 are connected to alternates ones of the straps 35 and 36. The bars 31 are crown supported at their ends from the conductive envelope structure 5.

. The lossy circuit termination 10 for the bar circuit 29 is provided by making the terminal bar elements 31 of the lossy material. Also the straps 35 and 36 in this section 10 may be made of the lossy material. Also the end conductive member portions 32 and 33 may be made of lossy material at 32' and 33' to provide further loss. If the straps 35 and 36 are segmented intermediate their points of connection to the bars 31 the bar circuit is transformed into a forward wave circuit that may be used in forward wave tubes of the type described with regard to FIG. 1. In this latter case the forward wave circuit would be terminated in the same manneras described for the bar circuit of FIGS. 15 and 16 except that the termination 10 would be placed at the opposite end of the cir cuit as shown in FIG. 1. Also both bar circuits may be provided with a short circuit wave reflective termination forming the sever 6 with the circuit length of the lossy termination being a quarter wavelength long to provide an effective short termination 10.

Referring now to FIG. 17 there is shown an alternative forward wave slow wave circuit embodiment useful for type 0 beam tubes and incorporating the circuit termination features of the present invention. More particular- 1y, there is shown a reactively loaded interdigital line slow wave circuit 39 similar to that shown in FIGS. 2-3 wherein the elongated conductors 9 and 11 are linear and wherein the interdigitated fingers 12 and 13 are centrally apertured to formed axially aligned rings for interaction with a beam of electrons 41 passed axially through the rings. The bi'furcating reactive loading slots 14 are made about twice as long as the fingers 12 and 13 to increase the interaction impedance of the circuit 39. The circuit 39 may be crown supported from a surrounding coaxial tubular conductive support wall structure 42, as shown, or supported from the wall 42 on insulators, not shown. The lossy termination 10 is formed by making one or more terminal periodic elements of the circuit 39 such as elements 12' and/or 13' of lossy material. In addition portions 9', 11 and 14 may be made of lossy material. As in the aforedescribe-d terminations 10 the terminal end of the circuit 39 may be shorted by a conductive structure such as a circuit sever, not shown. An effective short terminal section 10 is provided when the length of the circuit taken along the path of wave propagation on the circuit 39 is one quarter wavelength from the sever or shorted reflective end of the circuit 39 to the junction of the lossy material with the conductive material of the circuit 39. The circuit termination 10 for the ring circuit 39 is applicable in general to ring and ring-and-bar circuits.

Since many changes could be made in the above construction and many apparently widely ditferent embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a microwave periodic slow wave circuit, means forming a periodic slow wave circuit structure having a succession of periodic conductive elements for supporting traveling microwave energy thereon, means providing a lossy termination for such slow wave circuit at one end thereof, said lossy terminating means including at least one terminal periodic element for said periodic slow wave circuit structure, said terminal element having generally the same physical configuration as said conductive periodic elements of said slow wave circuit structure, said terminal periodic element having a circuit length of at least one period of said periodic slow wave circuit, and said terminal element being comprised in its entirety of a material having a resistivity greater than l()()0 l() Q centimeters, whereby the field configuration of the microwave energy propagating into said lossy termination means is not abruptly changed and whereby the termination requires only a short length of the slow wave circuit.

2. The apparatus of claim 1 wherein said periodic slow wave circuit is selected from the class consisting of, interdigital line, reactively loaded interdigital line, helices, helix coupled periodic elements, strapped bars, rings, and ring-and-bar circuits.

3. The apparatus of claim 2 wherein said lossy material of said terminating periodic element is selected from the class of carbon and carbon impregnated ceramic.

4. The apparatus of claim 1 including in combination, means forming a cathode electrode, said slow wave circuit including said terminating element being disposed adjacent said cathode and forming an anode structure to define an electronic interaction region in the space between said cathode electrode and said slow wave circuit, means for providing a magnetic field in said electronic interaction region directed transversely to the direction of the closest path between said cathode electrode and said slow wave circuit, and means for extracting wave energy from said slow wave circuit.

5. The apparatus of claim 2 including in combination, means forming a cathode electrode, said slow wave circuit including said terminating element being disposed adjacent said cathode and forming an anode structure to define an electronic interaction region in the space between said cathode electrode and said slow wave circuit, means for providing a magnetic field in said electronic interaction region directed transversely to the direction of the closest path between said cathode electrode and said slow wave circuit, and means for extracting wave energy from said slow wave circuit.

6. The apparatus of claim 5 wherein said lossy termination is provided at one end of said slow wave circuit and said output extracting means is disposed at the other end of said slow Wave circuit, and wherein the output is noise power.

7. The apparatus of claim 1 wherein said slow wave circuit includes a plurality of interdigitated ring portions forming periodic elements of said slow wave circuit, and wherein said terminating periodic element is a ring made of said lossy material, and including in combination means for forming and projecting a stream of electrons axially through said ring elements for electronic interaction with the fields thereof.

8. The apparatus of claim 1 including, means forming a wave reflector at the terminated end of said slow wave circuit, and wherein the distance taken along the path of wave propagation on said circuit from the start of said lossy material of said termination to said wave reflector means is approximately one quarter wavelength.

References Cited UNITED STATES PATENTS 1/1956 Dewey 3153.5 6/1966 Crapuchettes 3153.5 

1. IN A MICROWAVE PERIODIC SLOW WAVE CIRCUIT, MEANS FORMING A PERIODIC SLOW WAVE CIRCUIT STRUCTURE HAVING A SUCCESSION OF PERIODIC CONDUCTIVE ELEMENTS FOR SUPPORTING TRAVELING MICROWAVE ENERGY THEREON, MEANS PROVIDING A LOSSY TERMINATION FOR SUCH SLOW WAVE CIRCUIT AT ONE END THEROF, SAID LOSSY TERMINATING MEANS INCLUDING AT LEAST ONE TERMINAL PERIODIC ELEMENT FOR SAID PERIODIC SLOW WAVE CIRCUIT STRUCTURE, SAID TERMINAL ELEMENT HAVING GENERALLY THE SAME PHYSICAL CONFIGURATION AS SAID CONDUCTIVE PERIODIC ELEMENTS OF SAID SLOW WAVE CIRCUIT STRUCTURE, SAID TERMINAL PERIODIC ELEMENT HAVING A CIRCUIT LENGTH OF AT LEAST ONE PERIOD OF SAID PERIODIC SLOW WAVE CIRCUIT, AND SAID TERMINAL ELEMENT BEING COMPRISED IN ITS ENTIRETY OF A MATERIAL HAVING A RESISTIVITY GREATER THAN 1000X10-6 $ CENTIMETERS, WHEREBY THE FIELD CONFIGURATION OF THE MICROWAVE ENERGY PROPAGATING INTO SAID LOSSY TERMINATION MEANS IS NOT ABRUPTLY CHANGED AND WHEREBY THE TERMINATION REQUIRES ONLY A SHORT LENGTH OF THE SLOW WAVE CIRCUIT. 